This invention relates to liquid crystalline compositions and more specifically to liquid crystalline compositions having optically negative characteristics.
Recently there has been substantial interest in the discovery of more useful applications for the class of substances known as "liquid crystals." The name "liquid crystals" has become generic to liquid crystalline materials which exhibit dual physical characteristics, some of which are typically associated with liquids and others which are typically unique to solids. Liquid crystals exhibit mechanical characteristics, such as viscosities, which are ordinarily associated with liquids. The optical scattering and transmission characteristics of liquid crystals are similar to those characteristics ordinarily unique to solids. In liquids or fluids, the molecules are typically randomly distributed and oriented throughout the mass of the substance. Conversely, in crystalline solids the molecules are generally rigidly oriented and arranged in a specific crystalline structure. Liquid crystals resemble solid crystals in that the molecules of the liquid crystalline substances are regularly oriented in a fashion analogous to but less extensive than the molecular orientation and structure in a crystalline solid. Many substances have been found to exhibit liquid crystalline characteristics in a relatively narrow temperature range; but below such temperature ranges the substances typically appear as liquids.
Liquid crystals are known to appear in three different forms: the smectic, nematic and cholesteric forms. These structural forms are sometimes referred to as mesophases thereby indicating that they are states of matter intermediate between the liquid and crystalline states. The three mesophase forms of liquid crystals mentioned above are characterized by different physical structures wherein the molecules are arranged in a manner which is unique to each of the three mesomorphic structures. Each of these three structures is well known in the liquid crystal art.
Some liquid crystalline substances possess optically negative characteristics. Birefringence, also referred to as double refraction, is an optical phenomenon characteristic of some solid crystals and most liquid crystal substances. When a beam of unpolarized light strikes a birefringent substance it is split into two polarized components whose transverse vibrations are at right angles to each other. The two components are transmitted at different velocities through the substance and emerge as beams of polarized light. By the term "liquid crystalline substances which have optically negative characteristics," as used herein, is meant those for which the extraordinary index of refraction .eta..sub.E is smaller than the ordinary index of refraction .eta..sub.O. Cholesteric liquid crystal substances exhibit this property. For a detailed description of this phenomenon see Optical crystallography, Wahlstrom, 4th Edition, Wiley and Sons, Inc., New York.
The molecules in cholesteric liquid crystals are arranged in very thin layers with the long axes of the molecules parallel to each other and to the plane of the layers within each layer. Because of the asymmetry and steric nature of the molecules the direction of the long axes of the molecules in each layer is displaced slightly from the corresponding direction in adjacent layers. This displacement is cumulative over successive layers so that overall displacement traces out a helical path. A comprehensive description of the structure of cholesteric liquid crystals is given in "Molecular Structure and the Properties of Liquid Crystals," G. W. Gray, Academic Press 1962.
Cholesteric liquid crystals have the property that when the propagation direction of plane polarized or unpolarized light is along the helical axis thereof, i.e., when the light enters in a direction perpendicular to the long axes of the molecules, (neglecting absorption considerations), this light is essentially unaffected in transmission through thin films of such liquid crystals except for a wavelength band centered about some wavelength .lambda..sub.o where .lambda..sub.o = 2np with n representing the index of refraction of the liquid crystal substance and p the pitch or repetition distance of the helical structure. The bandwidth .DELTA..lambda..sub.o of this wavelength band centered about will typically be of the order of about ##EQU1## For light of a wavelength .lambda..sub.o, the cholesteric liquid crystal, under these conditions, exhibits selective reflection of the light such that approximately 50 percent of the light is reflected and approximately 50 percent is transmitted, assuming negligible absorption which is usually the case, with both the reflected and transmitted beams being approximately circularly polarized in opposite directions.
For light having wavelengths around .lambda..sub.o but not at .lambda..sub.o the same effect is present but not as pronounced. The transmitted light is not circularly polarized but is instead elliptically polarized. The cholesteric liquid crystals which exhibit this property of selective reflection of light in a region centered around some wavelength .lambda..sub.o are said to be in the Grandjean or "disturbed" texture. If .lambda..sub.o is in the visible region of the spectrum the liquid crystalline film appears to have the color corresponding to .lambda..sub.o and if .lambda..sub.o is outside the visible spectral region the film appears colorless.
Depending upon the intrinsic rotary sense of the helix, i.e., whether it is a right-handed or left-handed, the light that is transmitted in the region about .lambda..sub.o is either right-hand circularly polarized light (RHCPL) or left hand circularly polarized light (LHCPL). The transmitted light is circularly polarized with the same sense of polarization as that intrinsic to the helix. Thus, a cholesteric liquid crystal having an intrinsic helical structure which is left-handed in sense will transmit LHCPL and one having a helical structure which is right-handed in sense will transmit RHCPL.
Hereinafter these cholesteric liquid crystal substances will be identified, in order to conform with popular convention, by the kind of light which is reflected at .lambda..sub.o. When a film is said to be right-handed, it is meant that it reflects RHCPL, and when a film is said to be left-handed, it is meant that it reflects LHCPL.
A right-handed cholesteric liquid crystal substance transmits LHCPL essentially completely at .lambda..sub.o whereas the same substance reflects almost completely RHCPL. Conversely a left-handed film is almost transparent to RHCPL at .lambda..sub.o and reflects LHCPL. Since plane polarized or unpolarized light contain equal amounts of RHCPL and LHCPL, a cholesteric liquid crystal film is approximately 50% transmitting at .lambda..sub.o for these sources when the liquid crystal is in its Grandjean texture.
A further unique optical property of optically negative liquid crystal films is that contrary to the normal situation when light is reflected, such as by a mirror, where the sense of the circular polarization of the reflected light is reversed, this same phenomenon does not occur with light reflected by these liquid crystal films. The sense of the circular polarization of light reflected from these liquid crystal substances is not reversed but rather remains the same as it was before it came into contact with the liquid crystal substance. For example, if RHCPL having a wavelength .lambda..sub.o is directed at a right-hand film having .lambda..sub.o =2np it is substantially completely reflected and, after reflection, remains RHCPL. If the same light were to be directed on a mirror the reflected light would be LHCPL.
Because of these optical properties, optically negative liquid crystalline substances have been found to be highly advantageous for use in a number of varying applications. Copending patent applications Ser. No. 104,367 and Ser. No. 104,369, both filed Jan. 6, 1971, now U.S. Pat. Nos. 3,669,525 and 3,679,290 respectively disclose the use of such liquid crystalline materials in optical filter systems. The materials may be advantageously utilized in imaging methods such as are disclosed in copending application Ser. No. 821,565, filed May 5, 1969, now U.S. Pat. No. 3,652,148 and Ser. No. 867,593, filed Oct. 20, 1969, now U.S. Pat. No. 3,642,348. The thermal properties of these materials make them advantageous for use in thermometers, in detecting flaws in structural members, and in medical applications. Of course many other uses could be described but these should be sufficient to indicate the varied and important applications of optically negative liquid crystals.
In many of the applications cited above it would be desirable to have a liquid crystalline material which exists in the cholesteric mesophase at some temperature around room temperature (about 23.degree.C); although there are also applications where it is to be desired to have the liquid crystalline material in this optically active state above room temperature or below room temperature. To achieve a material having a particular desired operational cholesteric mesomorphic temperature as well as other desired properties, e.g. a particular pitch or electric field sensitivity, it has heretofore been the usual practice to form compositions which are made up entirely of cholesteric liquid crystals or combinations of cholesterics and nematic liquid crystals or combinations of cholesterics and smectic liquid crystals.
Furthermore it has been found that considerable versatility can be achieved with respect to cholesteric liquid crystals by mixing together combinations of right-handed and left-handed cholesteric liquid crystals. In such a mixture there is a composition at which the right-handed and left-handed components nullify each other to provide an infinite pitch. This technique also makes it possible to generally achieve a broader range of pitches than typically can be achieved by mixing together only right-handed or only left-handed materials. See Proc. ACS Symposium on Ordered Fluids and Liquid Crystals, Sept. 1969, page 463.
Many left-handed cholesteric liquid crystal materials are known; however to date only relatively few right-handed materials have been provided. Thus there exists a continuing need for optically negative liquid crystalline compositions, particularly those which are right handed.